For fishfinders that is one type of underwater detection devices, increasing in detection distance is demanded to detect a shoal of fish at a deeper location. In order to increase the detection distance, generally, a transmission pulse length is simply lengthened. However, if the transmission pulse length is lengthened, an echo reflected from a target object will also be longer, and the axial resolving power will be reduced. Thus, a method of increasing the axial resolving power may be adopted in which a transmission pulse is frequency-modulated, and a received echo signal and a replica waveform of the transmitted signal is correlation-processed to perform a pulse compression of the received signal. An underwater detection device that performs such a pulse compression process is disclosed in JP2005-249398(A).
However, in the underwater detection device that performs the pulse compression process, as shown in FIG. 10, false images referred to as “range side lobes” appear at positions before and after a main lobe, that shows a position of a detection target object, of a pulse-compressed signal.
When the range side lobes appear, especially due to a seabed, the false images may be mistakenly viewed by an operator as a target object, such as a school of small fish. In addition, an image of a shoal of fish located near the seabed may be difficult to view and a seabed depth may mistakenly be determined.